This invention relates to electrical probes, and particularly to microwave wafer probes.
Microwave wafer probes are well-known to the art, as exemplified by U.S. Pat. Nos. 4,697,143 and 4,849,689. Such probes have conductive probe tips which employ a dielectric substrate with printed planar conductors mounted thereon to form a controlled-impedance transmission line. One end of the probe tip is mounted in cantilevered fashion to a probe mount where it is attached to an electrical connector, while the other end of the probe tip contacts the device under test (DUT) to be probed. Contact between the probe tip and the DUT is accomplished by advancing either the probe mount or the DUT toward the other along a direction of advancement. When contact occurs, further advancement is used to establish a desired contact force, and the dielectric substrate bends to accommodate the advancement. As the advancement continues the contact force on the probe tip increases and, in the case of excessive advancement, the probe tip may become damaged due to excessive contact force. The substrate is mounted at an angle relative to the direction of advancement so that only the contact end of the probe tip actually contacts the DUT. Because of the angle, the contact end of the probe tip "skates" or slides forwardly on the DUT in a direction perpendicular to the direction of advancement as the contact force increases. A shallow probe tip angle, i.e. with the substrate extending generally normal to the direction of advancement, is usually necessary to minimize the magnitude of the skating, which is important because two or more probes are often used in conjunction on the DUT with their contact ends facing each other in close proximity (100 to 250 .mu.m typically) so that excessive skating could cause the two tips to collide and damage each other. Also, if a wafer prob tip were mounted at a steep angle, the contact force on the conductive strips would be higher, because the substrate cannot bend as advantageously as the contact force increases, and would be more likely to cause damage because the force would be more in a direction parallel to the conductive strips, inducing shear forces to dislodge the conductive strips from the substrate.
Because excessive advancement between the probe mount and the DUT is particularly likely to cause wafer probe tip damage with a steeply-angled probe tip, most wafer probes employ shallow-angled probe tips. However, shallow-angled probe tips make it difficult for such probe tips to reach down into chip interconnect packages without encountering interference from the surrounding package structure.
The possibility of probe tip damage also exists with probe types other than wafer probes, such as needle probes. Some probe types, as exemplified by U.S. Pat. Nos. 3,648,169, 4,251,772, and 4,888,550, and by German patent No. 205,759, feature probe tips resiliently connected to a probe mount by means of springs which flex to enable the probe tip to move relative to the probe mount as the probe mount and DUT are advanced toward each other. The springs cause a gradual increase in the contact force between the probe tip and the DUT as the proximity of the probe mount and DUT increases. However, such arrangements retain the problem of excessive advancement and resultant excessive contact force.
U.S Pat. No. 4,123,706 employs a pneumatic piston which tends to keep the contact force constant throughout a range of advancement, thereby providing some protection against excessive contact force. However, such protection is provided only over a narrow range of advancement, beyond which the piston is displaced to its limit so that it is no longer operative to limit the contact force. The piston's imposition of a constant contact force, rather than a gradually increasing contact force with increasing advancement, significantly limits skating of the probe tip relative to the DUT. However, skating is used as a visual indication of probe contact (viewable through a microscope), and therefore a gradual increase in contact force causing a controlled degree of skating is desirable. Also, such a pneumatic piston arrangement is far more complex and expensive than a spring system for movably mounting the probe tip relative to the probe mount